Optical waveguide circuit and method of fabricating same

ABSTRACT

An optical waveguide circuit having a waveguide core of a desired shape formed on a substrate and method of fabricating same. The optical waveguide circuit is manufactured in a manner such that light confinement portions having substantially the same refractive index as the waveguide core are arranged along one or both sides of the waveguide core. The waveguide core and the light confinement portions are formed simultaneously.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical waveguide circuit and methodof fabricating same.

2. Description of the Prior Art

In the field of modern optical communication, wavelength divisionmultiplexing (WDM) communication has been extensively investigated anddeveloped as means for drastically increasing the transmission capacity.Realization of the WDM communication requires development of an opticalwaveguide circuit that enjoys high wavelength branching performance or ahigh isolation characteristic.

FIG. 10 shows an example of an existing optical waveguide circuit. Anoptical waveguide circuit 20 shown in FIG. 10 is a wavelengthmultiplexing-branching filter of the Mach-Zehnder type, in whichwaveguide covres 20 b and 20 c, including a core and a clad, are formedon a substrate 20 a. In the optical waveguide circuit 20, a light beamwith wavelengths λ1 and λ2 that is projected from an optical waveguide,such as an optical fiber, onto a port P1 of the waveguide core 20 b isbranched into light beams with wavelengths λ1 and λ2 in an opticalmultiplexing-branching region Ab. The branch beams are guidedindividually to ports P2 and P3 of the waveguide cores 20 b and 20 c,and are emitted toward other optical waveguides such as optical fibersthat are connected optically to the ports P2 and P3.

In the optical waveguide circuit 20, light incident upon the clad, notupon the core, at the port P1 of the waveguide core 20 b and a radiationmode of the light generated in the optical waveguide circuit 20propagate in the clad as a clad mode. Thus, in a general region Aabehind the optical multiplexing-branching region Ab, the clad-mode lightlands on the waveguide cores 20 b and 20 c and leaks into the ports P2and P3.

If the clad-mode light is generated in the optical waveguide circuit 20,therefore, the isolation characteristic is lowered to cause cross talksand other adverse effects on the quality of optical communication.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an optical waveguidecircuit, capable of restraining clad-mode light from landing on awaveguide core or from leaking into other ports, and method offabricating same.

In order to achieve the above object, according to the presentinvention, there is provided an optical waveguide circuit having awaveguide core of a desired shape formed on a substrate. In this opticalwaveguide circuit, light confinement portions having substantially thesame refractive index as the waveguide core are arranged along one orboth sides of the waveguide core.

In the present specification, the phrase “light confinement portionshaving substantially the same refractive index as the waveguide core”implies that the respective refractive indexes of the light confinementportions and the waveguide core are equal or approximate to each other.Thus, based on the difference in specific refractive index between acore and a clad that constitutes the waveguide core, the width of eachlight confinement portion is adjusted to a value such that a maximumlight confinement effect can be obtained. For example, the width of eachlight confinement portion is set at about 1 μm or more, preferably at 3to 20 μm, and most preferably at 5 to 9 μm.

The waveguide core and the light confinement portion are arranged at adistance of 30 μm or more from each other.

Further, the waveguide core has a branching portion at which an opticalfilter is located across the waveguide core.

In order to achieve the above object, according to the presentinvention, there is provided a manufacturing method for the opticalwaveguide circuit having a waveguide core of a desired shape on asubstrate and light confinement portions arranged along one or bothsides of the waveguide core. In this manufacturing method, the waveguidecore and the light confinement portions are formed simultaneously.

According to the present invention, there may be provided an opticalwaveguide circuit and method of fabricating same, whereby clad-modelight can be restrained from landing on the waveguide core or fromleaking into other ports. According to the method of the invention,moreover, the waveguide core and the light confinement portions areformed simultaneously, so that the optical waveguide circuit can bemanufactured in simple processes at low cost.

The above and other objects, features, and advantages of the inventionwill be more apparent from the ensuing detailed description taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a first embodiment of an optical waveguidecircuit manufactured by a manufacturing method according to the presentinvention;

FIG. 2 is a sectional view for illustrating the distance between awaveguide core and a light confinement portion of the optical waveguidecircuit of FIG. 1;

FIGS. 3A to 3D are manufacturing process diagrams for illustrating themanufacturing method for the optical waveguide circuit of FIG. 1;

FIG. 4 is a plan view showing a modification of the optical waveguidecircuit of FIG. 1;

FIG. 5 is a plan view showing another modification of the opticalwaveguide circuit of FIG. 1;

FIG. 6 is a plan view showing still another modification of the opticalwaveguide circuit of FIG. 1;

FIG. 7 is a plan view showing a second embodiment of the opticalwaveguide circuit according to the invention;

FIGS. 8A and 8B are a plan view and a sectional view, respectively,showing a modification of the optical waveguide circuit of FIG. 7;

FIG. 9 is a plan view showing another modification of the opticalwaveguide circuit of FIG. 7; and

FIG. 10 is a plan view showing a conventional optical waveguide circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of an optical waveguide circuit according to the presentinvention and method of fabricating same will now be described in detailwith reference to the accompanying drawings of FIGS. 1 to 9.

An optical waveguide circuit 1 is based on a first embodiment, animprovement of a conventional optical waveguide circuit having aMach-Zehnder structure. As shown in FIG. 1, waveguide cores 1 b and 1 c,including a core and a clad, are formed on a substrate 1 a that isformed of a compound semiconductor material, such as silicon (Si) orgallium arsenide (GaAs), or some other material such as silica glass,sapphire, etc. Further, light confinement portions 1 d and 1 e, whichhave substantially the same refractive index as the waveguide cores 1 band 1 c, are arranged along the opposite sides of each waveguide core 1b and 1 c.

In the description to follow, regions indicated by broken lines Ab andAa corresponding to the waveguide cores 1 b and 1 c of the opticalwaveguide circuit 1 shown in FIG. 1 will be referred to as an opticalmultiplexing-branching region Ab and a general region Aa, respectively.

The light confinement portions 1 d and 1 e are formed in order to reducecross talks that are generated in the optical waveguide circuit 1. Asshown in FIG. 1, the light confinement portions 1 d extend along theoutside of the waveguide cores 1 b and 1 c, covering the overall lengthof the substrate 1 a. The light confinement portions 1 e are formedindividually on the longitudinally opposite end sides of the substrate 1a so as to extend along the inside of the waveguide cores 1 b and 1 c.

The optical waveguide circuit 1 with this construction is manufacturedby the following method. For example, fine particles of SiO₂-based glassthat is loaded with phosphorus, boron, etc. are deposited on thesubstrate by a flame hydrolysis reaction in the FHD (flame hydrolysisdeposition) method using an oxygen-hydrogen flame burner, and thedeposited glass particles are converted into transparent glass by ahigh-temperature heat treatment.

First, a lower clad layer 2 of a quartz-based glass material was formedon the substrate 1 a by the FHD method, as shown in FIG. 3A.

Thereafter, a core film 3 of a quartz-based glass material was formed bythe FHD method, as shown in FIG. 3B. The core film 3 has a refractiveindex higher than that of the lower clad layer 2 and an upper clad layer4 which will be mentioned later.

Then, the core film 3 was etched by the RIE (reactive ion etching)method, whereupon the waveguide cores 1 b and 1 c and the lightconfinement portions 1 d and 1 e were formed simultaneously, as shown inFIG. 3C.

Subsequently, the upper clad layer 4 of the quartz-based glass materialwas formed on the resulting structure by the FHD method, whereupon theoptical waveguide circuit 1 was completed, as shown in FIG. 3D. Themanufacturing method is not limited to the FHD method, and mayalternatively be sputtering, vacuum evaporation, or CVD (chemical vapordeposition). Further, the lower clad layer 2, core film 3, and upperclad layer 4 may alternatively be formed of an organic material such aspolyimide.

Since the refractive index of the light confinement portions 1 d and 1 eis higher than that of the lower and upper clad layers 2 and 4, light isconfined to the portions 1 d and 1 e. Thus, the light cannot leak outfrom these portions into the lower or upper clad layer 2 or 4. The lightconfinement portions 1 d and 1 e according to the present embodiment, inparticular, are made of the same material as the waveguide cores 1 b and1 c, since they are formed simultaneously with the cores 1 b and 1 c bythe RIE method.

A distance D between the waveguide core 1 b and its corresponding lightconfinement portion 1 d shown in FIG. 2, among the distances between thewaveguide cores 1 b and 1 c and the light confinement portions 1 d and 1e, is adjusted to an interval (=distance D) of about 30 to 70 μm or moresuch that no direct optical coupling occurs between the core 1 b and theportion 1 d.

In the general region Aa that is situated close to the opticalmultiplexing-branching region Ab shown in FIG. 1, as mentioned before,moreover, the light confinement portions 1 d and 1 e are located so thatthe distances between the waveguide cores 1 b and 1 c and the lightconfinement portions 1 d and 1 e are adjusted to the interval (=distanceD) of about 30 to 70 μm or more lest there be direct optical couplingbetween the waveguide cores 1 b and 1 c and the light confinementportions 1 d and 1 e. In this case, the light confinement portions 1 eare located even in the optical multiplexing-branching region Ab as longas the interval causes no direct optical coupling between them and thewaveguide cores 1 b and 1 c.

On the other hand, a width W (see FIG. 2) of each light confinementportion 1 d must be adjusted to a value such that the a maximum lightconfinement effect can be obtained, and is set at about 1 μm or more,preferably at 3 to 20 μm, and most preferably at 5 to 9 μm.

When a light beam with wavelengths λ1 and λ2 is projected from anoptical waveguide, such as an optical fiber, onto a port P1 of thewaveguide core 1 b, in the optical waveguide circuit 1 constructed inthis manner, it is branched into light beams with wavelengths λ1 and λ2in the optical multiplexing-branching region Ab. The branch beams areguided individually to ports P2 and P3 of the waveguide cores 1 b and 1c, and are emitted toward other optical waveguides such as opticalfibers that are connected optically to the ports P2 and P3.

In the optical waveguide circuit 1, light incident upon the clad layers2 and 4, not upon the waveguide core 1 b, at the port P1 and a radiationmode of the light generated in the optical waveguide circuit 1 (at awaveguide offset (mode scrambler), directional coupler portion,wavelength bent portion, etc.) propagate as leakage light beams in aclad mode.

In the optical waveguide circuit 1, however, the leakage light beams inthe clad mode get into and are confined to the light confinementportions 1 d and 1 e that are arranged individually along the oppositesides of the waveguide cores 1 b and 1 c. In the optical waveguidecircuit 1, therefore, the leakage light beams can be restrained fromlanding on the waveguide cores 1 b and 1 c in the general region Aabehind the optical multiplexing-branching region Ab or from leakingdirectly into the ports P2 and P3, so that a high isolationcharacteristic can be obtained.

As in the case of an optical waveguide circuit 5 shown in FIG. 4, lightconfinement portions 5 d and 5 e that are not coupled directly towaveguide cores 5 b and 5 c on a substrate 5 a may be arranged in thewhole hatched region that extends along the opposite sides of thewaveguide cores 5 b and 5 c. As in the case of an optical waveguidecircuit 7 shown in FIG. 5, moreover, a light confinement portion 7 d maybe located along each side of each of waveguide cores 7 b and 7 c on asubstrate 7 a so as to cover the overall length of the substrate 7 a. Inthe optical waveguide circuits 5 and 7 constructed in this manner,clad-mode light can be restrained from leaking into other ports.

Further, the light confinement portions need not be arranged throughoutthe length of the waveguide cores only if they can restrain theclad-mode light from leaking into other ports. As in the case of anoptical waveguide circuit 9 shown in FIG. 6, for example, a lightconfinement portion 9 d may be located along each side of each ofwaveguide cores 9 b and 9 c on a substrate 9 a so as to extend close tothe opposite ends and away from the cores 9 b and 9 c near the ends.Thus, it is necessary only that the length of each light confinementportion 9 d be settled so that cross talks between the waveguide cores 9b and 9 c can be prevented securely.

An optical waveguide circuit according to a second embodiment of theinvention and method of fabricating same will now be described withreference to FIGS. 7 to 9.

As shown in FIG. 7, an optical waveguide circuit 10 comprises Y-shapedwaveguide core 10 b, including a core and a clad and formed on asubstrate 10 a that is formed of a compound semiconductor material, suchas silicon (Si) or gallium arsenide (GaAs), or some other material suchas silica glass, sapphire, etc., and a plurality of light confinementportions 10 c, which have substantially the same refractive index as thewaveguide core 10 b and are arranged along the opposite sides of thecore 10 b. Further, the optical waveguide circuit 10 is formed with agroove 10 d that extends in the crosswise direction of the substrate 10a at the branching portion of the core 10 b. A dielectric multi-layerfilter 11 is located in the groove 10 d. In the optical waveguidecircuit 10, the waveguide core 10 b and the light confinement portions10 c are arranged at intervals of about 30 to 70 μm lest there be directoptical coupling between them.

The filter 11 is a conventional optical filter that transmits a lightbeam with the desired wavelength λ1 and reflects a light beam with thedesired wavelength λ2.

The optical waveguide circuit 10 constructed in this manner, like theoptical waveguide circuit 1 according to the foregoing embodiment, wasmanufactured by simultaneously forming the waveguide core 10 b and thelight confinement portions 10 c by the RIE method.

In the optical waveguide circuit 10 constructed in this manner, a portP1 of the waveguide core 10 b serves as a common port for the incidenceand emission of the light beams with the wavelengths λ1 and λ2, a portP2 as a port for the incidence and emission of the light beam with thewavelength λ1, and a port P3 as a port for the incidence and emission ofthe light beam with the wavelength λ2.

In the optical waveguide circuit 10, therefore, the light beam with thewavelength λ1 projected through the port P2 onto the waveguide core 10 bis guided to the port P1 through the filter 11. In the optical waveguidecircuit 10, in this case, clad-mode light is generated at the branchingportion or the port P2 of the waveguide core 10 b. The light beam withthe wavelength λ1 is propagated to the port P3 for the incidence andemission of the light beam with the wavelength λ2, in particular,whereby the isolation characteristic is lowered.

On the other hand, the light beam with the wavelength λ1, out of thelight beams with the wavelengths λ1 and λ2 projected through the port P1onto the waveguide core 10 b, is transmitted through the filter 11 tothe port P2, while the light beam with the wavelength λ2 is reflectedand guided to the port P3. In the optical waveguide circuit 10, in thiscase, clad-mode light is generated in like manner at the port P1 of thewaveguide core 10 b and the branching portion corresponding to thefilter 11. Further, the light beams are scattered or reflected (e.g.,the light beam with the wavelength λ1 is partially reflected withoutbeing entirely transmitted through the filter 11) at the branchingportion or the filter portion and propagated to the port P3, therebylowering the isolation characteristic.

In the optical waveguide circuit 10, however, a plurality of lightconfinement portions 10 c are arranged along the opposite sides of thewaveguide core 10 b. In the optical waveguide circuit 10, therefore, theclad-mode light is confined to the light confinement portions 10 c, sothat it can be restrained from leaking into other ports, thus ensuring ahigh isolation characteristic.

In the optical waveguide circuit 10, regions to be mounted with opticaldevices, such as a semiconductor laser (LD), photodiode (PD), etc., maybe formed on the substrate 10 a by etching. In this case, electrodes andsolder patterns are formed for the optical devices. As shown in FIGS. 8Aand 8B, the optical waveguide circuit 10 may alternatively be designedso that shading grooves 10 e are formed in- and outside a plurality oflight confinement portions 10 c on the substrate 10 a by etching. Asshown in FIG. 9, moreover, shading grooves 10 f may be formed on thoserespective end portions of a plurality of light confinement portions 10c which are situated on the side of the groove 10 d. In these cases, thesubstrate 10 a is exposed through the shading grooves 10 e or 10 f. Inthe optical waveguide circuit 10 constructed in this manner, clad-modelight can be restrained from leaking into other ports, so that a higherisolation characteristic can be obtained.

What is claimed is:
 1. An optical waveguide circuit having at least onewaveguide core of a desired shape formed on a substrate, wherein atleast one light confinement portion having substantially a samerefractive index as each side waveguide core is arranged along at leastone side of each side waveguide core at an interval from each sidewaveguide core.
 2. An optical waveguide circuit having at least onewaveguide core of a desired shape formed on a substrate, wherein atleast one light confinement portion having substantially a samerefractive index as each said waveguide core is arranged along at leastone side of each said waveguide core, and wherein a width of each saidlight confinement portion ranges from 3 to 20 μm.
 3. The opticalwaveguide circuit according to claim 1, wherein each said waveguide coreand each said light confinement portion are arranged at a distance of 30μm or more from each other.
 4. The optical waveguide circuit accordingto claim 2, wherein each said waveguide core and each said lightconfinement portion are arranged at a distance of 30 μm or more fromeach other.
 5. An optical waveguide circuit having a waveguide core of adesired shape formed on a substrate, wherein at least one lightconfinement portion having substantially a same refractive index as saidwaveguide core is arranged along at least one side of said waveguidecore, and wherein said waveguide core has a branching portion at whichan optical filter is located across the waveguide core.
 6. The opticalwaveguide circuit according to claim 5, wherein a width of each saidlight confinement portion ranges from 3 to 20 μm.
 7. The opticalwaveguide circuit according to claim 5, wherein said waveguide core andeach said light confinement portion are arranged at a distance of 30 μmor more from each other.
 8. The optical waveguide circuit according toclaim 6, wherein said waveguide core and each said light confinementportion are arranged at a distance of 30 μm or more from each other. 9.The optical waveguide circuit according to claim 5, wherein said opticalfilter transmits a light beam with a first wavelength and reflects alight beam with a second wavelength.
 10. The optical waveguide circuitaccording to claim 6, wherein said optical filter transmits a light beamwith a first wavelength and reflects a light beam with a secondwavelength.
 11. The optical waveguide circuit according to claim 7,wherein said optical filter transmits a light beam with a firstwavelength and reflects a light beam with a second wavelength.
 12. Theoptical waveguide circuit according to claim 8, wherein said opticalfilter transmits a light beam with a first wavelength and reflects alight beam with a second wavelength.
 13. A method of manufacturing anoptical waveguide circuit having at least one waveguide core of adesired shape on a substrate and at least one light confinement portionhaving substantially a same refractive index as each said waveguide corearranged along at least one side of each said waveguide core, saidmethod comprising simultaneously forming each said waveguide core andeach said light confinement portion.